Issue

Yesterday’s purity isn’t enough

12/01/2007

Continually scaling circuit features have led to tightened process requirements, including the purity levels of gas and liquid chemicals used in semiconductor manufacturing. How will the industry respond to the contamination control challenges posed by gas and chemical distribution?

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By Hank HoganFor gas and liquid distribution systems in semiconductor cleanrooms, a high level of purity is a necessity, one that’s becoming more urgent all the time. Shrinking semiconductor features are the overarching factor.

Hugh Gotts, director of research and development for Air Liquide Electronics U.S. – Balazs Analytical Services (Fremont, CA; a subsidiary of gas supplier Air Liquide), notes that smaller features mean fewer and fewer atoms constitute a process film layer. “If you have a layer that’s made out of five or ten atoms on average, and if you have one layer of atoms as a contaminant, that would certainly be too much,” he says.

The smaller width, length, and height of such critical circuit elements as transistor gates in today’s state-of-the-art 45 nm node show up in the requirements spelled out in the International Technology Roadmap for Semiconductors. A look at the industry consensus document reveals what purity levels are needed now and what will be required in the future.

For example, ultra-pure water today must have fewer than 0.2 particles above critical size per milliliter. That’s unchanged over the next half decade, but the critical size scales with the node, since, as a rule of thumb, the size of a killer particle is half that of the node. This means the size of allowable particles will drop 30 percent over the next five or so years, a test for filtration technology. Similar situations apply to liquid chemicals such as the acid hydrogen fluoride or the base ammonium hydroxide.

As for gases, nitrogen is supposed to drop from 5 ppb trace contaminants now to less than 1 ppb in 2010. Other gases such as the corrosive etchant boron trichloride likewise face tightened trace contaminant requirements.

So how is the industry confronting these and other contamination control challenges in ultra-pure gas and chemical distribution?

Millions and millions cycled

Ultra Clean Technology (Menlo Park, CA) is one of the biggest providers of critical subsystems, including fluid and gas delivery systems to the semiconductor and flat panel industries, notes vice president of technology and CTO Sowmya Krishnan, PhD. The company constructs these systems at one of four sites around the globe, each having several thousand square feet of cleanroom capacity. Cleanliness levels range from Class 1 (ISO 3) to Class 1000 (ISO 6), with the goal being to produce systems that are virtually free of contamination and particulates.

Krishnan sees several technical trends affecting fluid distribution systems. She says achieving the required cleanliness and purity isn’t a chief concern. Instead, the challenge now lies elsewhere. “It’s moved to areas such as the reliability of the gas delivery system,” says Krishnan.

Figure 1. A Balazs technician uses a high-resolution inductively coupled plasma???mass spectrometer (ICP-MS) for ultra-low level analysis of contamination in IC processing chemicals and pure water used in the manufacturing process. A full wafer desorber (background) is used to analyze airborne contamination after collection. Photo courtesy of Air Liquide Electronics U.S. ??? Balazs Analytical Services.

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Process requirements provide an explanation for such demands. Atomic layer deposition, for example, is being more and more widely used. Critical process layers are fabricated almost literally atom by atom. In the course of putting down one monatomic layer after another, tool process valves will cycle many, many times every few milliseconds. Over the course of its lifetime, an atomic layer deposition tool may open and close valves tens of millions of times. Each cycle has to be like the last, so what is metered out in the beginning is the same as what is dispensed at the end. As a result, the valves and other components of the distribution system must be extremely reliable.

Reliability and accuracy specifications are getting tighter on other metering devices, such as flow controllers. At one time, a device would be specified as having 1 percent of full flow accuracy. So a 100 standard cubic centimeter per minute (SCCM) flow controller would have an accuracy one-hundredth of that.

Now, the requirement is for the same percentage but with respect to the process flow set point. Thus, if the set point were 20 SCCM flow, the flow controller would be expected to be accurate to 0.20 SCCM flow, an effective five-fold increase in specified accuracy.

Process changes are having an impact on other areas as well. “Corrosion issues are showing up quite a bit. That could be because customers are using higher-concentration chemistries,” says Krishnan of Ultra Clean Technology.

She explains that manufacturers are now fabricating denser circuitry with smaller feature sizes while trying to maintain the same tool throughput. This has forced them to make changes in the chemistries and go with higher concentrations of corrosive gases and fluids. As a consequence, distribution system manufacturers have had to prepare for this possibility through manufacturing and material changes, as well as paying attention to such issues as interactions between chemicals.

Bigger fabs require more gas

While circuit feature sizes have been growing smaller, the fabs they’re made in are growing larger. According to figures from semiconductor industry analysis firm IC Knowledge (Georgetown, MA), the average size of a 300 mm fab in terms of wafer capacity has grown from 14,300 per month in 2004 to nearly more than 23,500 this year. It’s projected to grow to almost 36,200 in 2012. President Scotten Jones says this is a consequence of the advent of megafabs with 100,000 wafers per month capacity. For companies doing business in gas and chemical distribution, the effect is one of increasing demand.

Shrikar Chakravarti, manager of electronics supply systems research and development at gas supplier Praxair, Inc. (Danbury, CT), notes that gases are classified as either bulk or specialty/process. The former include nitrogen and oxygen, both of which can be made out of the air by plants on site. The demand clearly needs to be high enough to warrant the investment.

Specialty gases include ammonia, silane, and nitrogen trifluoride, to name a few. Unlike bulk gases, they’ve typically been manufactured off site, stored in cylinders, trucked to a fab, and then installed. As might be imagined, for large fabs with high rates of consumption, this approach can mean moving a lot of cylinders around. The handling increases the chance for contamination to be introduced into the distribution system.

In response to this and other issues, Praxair introduced a bulk specialty gas supply system and controller. Instead of using a cylinder, such systems use other larger capacity options: possibly a ton container, which is like a giant cylinder, or a tube trailer. The system’s controllers ensure reliability. “The emphasis is on making sure we have reliable and safe delivery systems that provide the customer the gas at the flow and purity that they need,” says Chakravarti.

Ringing in less contamination

Because required purity levels for all gases are increasing, measurement can be a challenge. The issue can be one of technical capability and cost.

Tiger Optics (Warrington, PA) is a company that’s working on the measurement side of the problem. The company’s products are based on continuous-wave cavity ring-down spectroscopy. (Tiger Optics has licensed a patented continuous-wave laser technology from Princeton University.) In this technology, a laser enters a chamber containing the sample gas to be measured. Highly reflective mirrors send the laser back and forth across the gas, creating a long path length and maximizing absorption. With the laser shuttered or diverted, a detector measures the light energy. The time it takes to fade, or ring down, to a low enough level provides a sensitive measure of the purity of the gas.

The ring-down only takes milliseconds, and the technology can measure trace contaminants in gas in the parts per billion and below. “Right now we go down to 200 parts per trillion of moisture in inert gases. That’s pretty good for this node and maybe the next,” says Keith Barnes, director of U.S. sales in ultra-high purity and semiconductor applications for Tiger Optics.

These systems are in-line constant measurement devices, which spot contaminants because they shorten the ring-down time. Care must be taken not to introduce moisture or other undesirables when readings are made.

A decade ago, the technology wasn’t available. That changed when it was proven that inexpensive continuous-wave lasers could be used. The lasers in the Tiger Optics devices came out of the telecom industry, which means they operate in the infrared at up to about 2,000 nm.

Figure 2. Reliability and accuracy of chemical distribution systems has become critical in controlling semiconductor processes. Shown here: The field service director at Ultra Clean Technology consults with a client on a service request. Photo courtesy of Ultra Clean Technology.

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According to Barnes, lasers that operate at longer wavelengths—3,000 nm or so—would extend the technique to lower and lower detection limits. The problem is that there is no large market driver to push the development of less expensive longer-wavelength lasers. That makes it economically impractical to actually produce more sensitive devices. However, that situation could change if demand for longer-wavelength lasers were to increase significantly.

One benefit of continuous monitoring technologies could be to help resolve yield issues. Although a relatively high level of contaminants may kill a device, a lower concentration may just make it perform badly. Thus, if a yield crash occurs, one culprit could be a new gas bottle or a point-of-use purifier that has a problem. With continuous monitoring, fab managers would know about these issues before bad die started coming out of the end of the line. Lower operating costs could result, since it might be possible to run purifiers for a longer time before changing them.

Air and water

The smaller size of allowable particulates has a ripple effect on gas and chemical distribution systems. Bob Wadja, a senior product marketing manager at filtration and ultra-pure material handling company Entegris (Chaska, MN), says that chemical filtration systems, for example, are shifting down to 30 nm (0.03 µm), particle retention ratings for critical chemicals with the transition to the 45 nm node. In some cases even 20 nm (0.02 µm) capability is required. That’s smaller than the previous 50 nm (0.05 µm) standard.

That change might be positive for contamination control, but it also may present another challenge. As the filter retention rating decreases, typically increased pressure is needed to maintain the same flow rate. One solution is to improve the filter material, which Entegris achieved with its TorrentoTM product. Wadja says the new material can achieve 20 nm filtration that matches the flow rate of the older 50 nm media at the same pressure.

The new filtration media must, of course, meet increasing cleanliness requirements with regard to metals, organics, and other trace contaminants. Contamination specifications depend upon the process technology and the part being manufactured. NAND flash memory, for instance, is reportedly very sensitive to metal contaminants. “We’re seeing an increasing emphasis being placed on the cleanliness of liquid filters for a variety of applications,” says Wadja.

Figure 3. Bulk specialty gas supply systems with greater capacity and built-in controllers, such as Praxair’s SureflowTM, can reduce the risk of contamination due to handling and installation. Photo courtesy of Praxair.

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Dave Ruede, head of the gas micro-contamination group at Entegris, sees his company’s products as having two main jobs. One is to not add particles or contaminants. The second is to reduce particle loading of materials passing through. The reason for this focus on both functions is that the purifiers and filters process fluids used in the cleanest and most sensitive manufacturing areas.

“We have to make sure that we don’t have any of those killer defects coming out because we’re putting these materials very close to, if not onto, the wafer,” sums up Ruede.

He notes that when it comes to molecular contamination, airborne or not, what was acceptable in the past may not be now or in the future. A prime example is ammonia levels in cleanroom air. Photolithography chemistries widely used today can be poisoned by levels in the parts per billion, far below any regulatory limit for humans. At one time, though, such levels wouldn’t pose a problem for semiconductor processing, either. A change in the lithography process, specifically a switch to a new light source and chemistry, made low ammonia concentrations an issue of concern.

A similar sort of change has just taken place as manufacturers have begun to implement immersion lithography, creating a need for high-purity compressed dry air, which is used to purge the final exposed lens element. With today’s scanners consuming 270 to 390 L of clean dry air per minute, demand is only going to grow. Next year’s scanners are expected to gulp down the gas at four times that rate.

To help satisfy the need for highly pure dry air, Entegris has just released a new addition to its line of compressed dry air purifiers. Thanks to proprietary changes made by the company in the filtration technology and media, Ruede says the facility load for these latest products has dropped, while the contaminant level is the parts per trillion range.

Immersion lithography isn’t just about air, though. The pre-eminent fluid is the water that comes into contact with both wafer and lens. That water must be controlled for temperature, flow, dissolved gases, particles, and molecular contaminants. Without such control, miniscule fluctuations in water properties can cause changes in its optical performance, thereby altering the final printed pattern.

The next frontier?

Another big change, of course, is the switch from silicon dioxide transistor gates to those based on such previously unused metals as hafnium. The chemistry of silicon dioxide and its interaction with gases and liquids is well known. Just as well understood is how those process fluids interact with one another and what to watch out for to avoid side reactions that could produce particles and other contaminants.

The same can’t be said for the new structures and chemistries. At the semiconductor research and development foundry Advanced Technology Development Facility (ATDF) (Austin, TX), the staff has experienced some of the effects of the new processes. “The use of a larger set of process-based materials is leading to us having to pay a lot more attention to what’s going on at critical interfaces and any environment the wafer sees,” says ATDF front end processes module manager David Dyer.

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Senior chemical engineer Mac Willmert of fab engineering firm CH2M Hill (Englewood, CO) notes an unexpected facilities impact that arises from monitoring and analysis of gases. Optical instruments require a stable base with vibration isolation akin to that used for lithography steppers. Finding or creating such a quiet spot on the floor next to a process tool can be difficult.

In looking out to the future, Willmert believes the facilities action, and perhaps much of the new contamination focus, won’t be in the traditional semiconductor cleanroom but in the area that immediately follows—assembly and test. That has been going through a process of clean-up for years, but the trend may be accelerating due to processing changes.

In order to achieve required device performance, the semiconductor industry is going into the third dimension. The plan is to stack die atop one another and electrically connect them by etching vias and flowing in a conductor.

Figure 5. New developments in filtration technology enable equipment suppliers to provide advanced compressed dry air purifiers, such as the Aeronex Z2. Photo courtesy of Entegris.

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That push, if successful, will add etching, cleaning and bonding to assembly and test. That could drive some new chemistry requirements and new fluid distribution system needs. Because it will come very late in the process, after the die are done, all the new processing, as well as standard testing to make sure the die are functional, will have to be done without impacting yield or performance. As Willmert says, “It may impose cleanliness requirements on testing that just haven’t been there to date,” posing a whole new set of challenges for the semiconductor industry to conquer.